1. Field of the Invention
The present invention relates to a non-dispersive type infrared analyzer which can simultaneously determine the concentration of two kinds of components contained within a fluid sample.
2. Description of the Prior Art
A prior art analyzer is shown in FIG. 1. Such an analyzer adopts a "cross modulation system" in which the zero fluid and the sample fluid are alternately provided in two cells 2 and 2' by changing over a passage-changing over member 6 at a definite period. In addition, a pneumatic detector 7 having light-receiving chambers 7a and 7b and a pneumatic detector 8 having light-receiving chambers 8a and 8b are arranged in series with respect to light-passages through which light generated by light sources 1 and 1' pass through cells 2 and 2'; an analyzer of this type has an advantage in that it is relatively immune to changes in the ambient temperature and thermal fluctuations due to the use of a pneumatic detector having two light-receiving chambers in addition to such advantages as the absence of zero-drift and a good S/N ratio. However, such prior art analyzers also have disadvantages in that they are influenced by the mutual interference of the two components whose concentration is to be determined and by the interference of the other components contained within the sample fluid. In particular, in the case when the concentration of one of the components to be determined is lower than that of another component, there is the possibility that the error due to the interference in the determination of the concentration of the low-concentration gas is particularly large. Furthermore, in the high concentration-low concentration case described above, an analyzer as shown in FIG. 1 is not suitable for accurately determining the concentration of both components since it uses cells 2 and 2' having the same length.
That is the absorption in the cell, in which the sample fluid is provided, is changed in dependence upon the product of the concentration multiplied by the cell-length along a curve similar to that of Lambert-Beer's law, as shown in FIG. 2. The smaller the value of the absorption is, the better the linearity is. Consequently, the selection of the optimum cell-length for the low-concentration component whose concentration is to be determined leads to a curvature of the calibration curve and a wrong scale accuracy of the instrument when used in the determination of the concentration of another high-concentration component. Thus, a high linearity for the determination of the concentration of the high-concentration component which is attained by decreasing the cell-length leads to a low S/N ratio in the determination of the concentration of the low-concentration component.